"Optical Illusion" joke

A man and his wife had been stranded on a deserted island for
many years. The morning following a bad storm, a new guy washes
up on the shore. The new guy and the wife are VERY attracted to
each other right away, but they realize that certain protocols
will have to be observed. The husband, oblivious to the
pheremones floating around, is just glad to have someone new to
talk to. "This is wonderful! Now we'll be able to have three
people doing 8-hour shifts in the watchtower instead of two
people doing 12-hour shifts." The new man is only too happy to
help, and in fact volunteers to do the first shift.
He climbs up the tall tower and stands watch, scanning the ocean
horizon for any ships. Soon the husband and wife start placing
stones in a circle in order to make a fire to cook supper. The
new man yells down: "Hey, no screwing!" They look at each other
and yell back: "We're not screwing!" A few minutes later, they
start to put driftwood into the stone circle. Again the new man
yells down: "Heeey, no screwing!" Again they yell back, "We're
not screwing!" Later they are putting palm leaves on the roof of
the shack to patch leaks. Once again the new man yells down from
high above: "Hey, I said no screwing!!" They yell back, "And we
said we're not screwing!!"
Finally the shift is over and the new man climbs down from the
tower and the husband starts to climb up. By the time he gets
half-way up, his wife and the new man are already screwing their
brains out. Once at the top, the husband turns around and looks
down and says to himself: "Son-of-a-gun. From up here it DOES
look like they're screwing."